But I tried to come up with a list of my favorites:
Oblivion Mod Manager: helps keep the mods organized - I didn't use enough mods to make this totally worthwhile, but it helped keep things orderly.
Oblivion's Real Estate: this is a site that is dedicated to creating new houses and locations - very cool (been a while since I visited the site)
Drop Lit Torches in Combat: lets you drop a torch and keep the light so you don't have to fight in the dark
(Forgotten Name): I had a mod that had a house with tons of storage space, enchanting and spell making altars, lots of plants to make potions, and a fully stocked potion making room. Sorry I can't remember the name.
Mighty Magick: My favorite mod. Yes, it broke the game, and made me a force to be reckoned with. I love magic-user characters. I mean, there is a reason nobody messes with hi level magic users, right? I just made it more apparent.
PJs Spell Compendium: This mod adds a ton of different spells. I didn't use very many of them, but it added some variety.

That's what I can remember looking over old notes from the game. I think I had a couple mods that added new weapons as well, and I used a mod that took off the quest markers - sorry, I can't remember the name of the that one.